Directional Solidification of Alloys in Systems Containing a Liquid Miscibility Gap
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DIRECTIONAL SOLIDIFICATION OF ALLOYS IN SYSTEMS CONTAINING A LIQUID MISCIBILITY GAP
R. N. GRUGEL, T. A. LOGRASSO and A. HELLAWELL Department of Metallurgical Engineering, Michigan Technological University, Houghton, Michigan, USA
ABSTRACT The problem considered is the unidirectional growth of alloys close to monotectic composition and within the miscibility gap when the direction of gravity is down the temperature gradient (solidification upwards) or up the temperature gradient (solidification downwards). The systems Al-In and Al-Bi are taken as examples. With solidification upwards it is shown that bulk liquid composition adjusts to that of the monotectic while in the reverse situation, the bulk liquid gradually rises. In the former case it is possible to grow aligned fibrous structures of monotectic composition but in the latter the microstructures are irregular and globular.
INTRODUCTION AND PROCEDURE The monotectic reaction, Ll-Solid + L2 , occurs in many systems and in most cases the form of the free energy composition curves is such that the monotectic liquid, L 1 , occurs at relatively low levels and the volume proportion of the L2 phase is small - generally less than 10% (Fig. 1). In most metal systems it also happens that the L2 phase has a higher density than that of L It has been of interest to investigate the possibility of producing aligneA composites of variable volume fraction by solidifying hypermonotectic alloys with the miscibility gap, and in view of the density differences it has been relevant to vary the solidification direction with respect to gravity. The study is therefore comparable to similar studies of off-eutectic alloys (1) with solidification vertically upwards (traction downwards, g down the temperature gradient) or downwards (traction upwards, g up the temperature gradient) (2). In the Al-In system the monotectic composition is %17.5 wt.% In (volume fraction "'5% In); in the Al-Bi system the monotectic is at "4 wt.% Bi (volume fraction is "'1% Bi). In both systems alloys of monotectic composition can be solidified upwards to produce aligned fibrous structures of L 2 in a matrix of 1 solid, SI, at growth rates below %5 pm 8-1 with temperature gradients 515K mm(3). In the present work alloy compositions have been examined in the range 17"5 - 30 wt.% In and 4 - 8 wt.% Bi. Experimental procedure has been the same as in previous work (3) except for alloy preparation. Previously, molten alloy was drawn into refractory tubes under vacuum and allowed to solidify slowly in a furnace. This procedure, while ensuring fairly complete filling of specimen tubes, inevitably allows the more dense liquid to segregate to one end of specimens so that the bulk initial composition is lower than that intended and closer to the monotectic composition. Therefore, to ensure that hypermonotectic compositions were closer to those chosen, alloys were chill cast from above the miscibility gap and swaged to a diameter which would just allow material to be pushed into the refractory tubes. 300mm lengths of s
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